U.S. Pat. No. 5,858,199, hereafter referred to as the Hanak patent, contains the description of apparatus and method for a water deionization process named Electrocoriolysis, also referred to as the ELCOR.TM. process. The background of the invention that appears in the Hanak patent, contains a detailed description of the electrolytic and the electrostatic modes, which is also relevant to the instant invention. It should be noted that the term `electrostatic` in this description refers to a deionization process assisted by gravitational or centrifugal forces, while the term `capacitive` refers to a deionization process not involving said forces; otherwise both the electrostatic and capacitive processes involve capacitive charging and discharging of the electrodes. Additional background information, which applies to the ELDYN mode, follows.
While conducting tests using a dynamic Electrogravitational (EG) deionization device operating in the electrostatic mode, a new, previously unknown mode, co-existing and competing with the electrostatic mode has been discovered. As stated above, this new mode was named electrodynamic mode, or ELDYN mode. It was observed that unlike in the capacitive method of prior art [References 1, 2, 3], deionization and enhancement in the electrostatic mode were occurring simultaneously and continuously with the newly discovered ELDYN mode, solely by the combined action of an electrostatic field and gravitational force. On account of the fact that this new phenomenon had an implication of potential large gains in the throughput and energy efficiency of the water treatment process, an extensive examination of the results was undertaken to determine the mechanism of the ELDYN mode.
Evidence for the Existence of the ELDYN Mode of Deionization
The preceding test data indicate that the ELDYN mode occurs simultaneously with the electrostatic mode. The two appear to be competing processes. The occurrence of the ELDYN mode has been inferred from the mechanism previously known to be taking place in the capacitive mode of prior art [References 1, 2, 3], from three observations obtained in the study of EG deionization in the electrostatic mode, and from the first successful deionization using the ELCOR.TM. process operating in the electrostatic mode.
(a) Mechanism of Deionization in the Capacitive Method
Oren and Soffer [Ref. 1, 2], in describing their deionization process by `electrochemical parametric pumping` that appears to be the original version of the capacitive method of deionization, observe that "almost all of the electric charge is directed to change NaCl concentration." Farmer [Ref. 3], in his patent on a capacitive method of deionization, reported that deionization occurs only during charging, and enhancement occurs only during discharging. There was no provision in either case for Earth's gravity to assist deionization.
(b) Evidence from Simultaneous Deionization and Enhancement
The first piece of evidence, from FIG. 10 in the Hanak patent, reproduced herein as FIG. 1, is that during a voltage pulse commencing at .about.1350 s and ending at .about.4000 s, as well as during subsequent pulses, a high rate of deionization and enhancement were taking place simultaneously during the charging process, shown by the increasing voltage. Whereas deionization is expected during charging, enhancement is not expected until the polarity reversal, when capacitor discharge and the release of accumulated ions occur, as described in (a) above. We postulate that the simultaneous occurrence of enhancement is the consequence of the presence of the electrical double layer at the electrode surfaces, shown in FIG. 2 [Ref. 4]. The diffuse layer in the double layer contains elevated concentration of solvated ions having polarity opposite that of the electrode, rendering the solution in it more dense. Under the influence of gravitational or centrifugal force, the diffuse layer slides in the direction of this force, like an avalanche, along the surface of the electrode, while being held close to it by electrostatic force, resulting in the observed enhancement at the bottom of a stationary cell or the outer periphery of a rotating cell. The water molecules between the electrode and the diffuse layer act as a lubricant for this sliding motion. At the same time, the partially depleted solution between the electrodes moves in the direction opposite to the gravitational or centrifugal force to cause the observed depletion at the top of a stationary cell or near the hub of a rotating cell. This process constitutes a `leaky` capacitor. Current must be constantly supplied to make up for the ions removed from the electrode surfaces. This current is in addition to the capacitive charging current.
This postulated mechanism for the ELDYN mode implies that in the ELCOR.TM. process, in which centrifugal force is used, which can be made much greater than the gravitational force, the diffuse layer will be removed by the sliding action at a much greater rate, causing the ELDYN mode to predominate over the electrostatic mode.
(c) Evidence from the Duration of the Current Pulse
The second piece of evidence is the duration of the current pulse at the same, constant level of current, I, for different chemical species. This mode of charging is referred to as the `current step` method, in which the potential, E, across the electrodes increases linearly with time, t, according to the equation: EQU E=I(R.sub.s +t/C.sub.d), Eq. 1
where R.sub.s is the resistance in the electrolyte [Ref. 4] and C.sub.d is the double-layer capacitance. With the same set of electrodes, the charging time, t, should be the same to reach the same potential, E. Yet, in FIG. 9 in the Hanak patent, reproduced here as FIG. 3 and in FIG. 1, the average length of the current pulses were 870 s. (0.24 h) and 2390 s. (0.66 h) for CaCl.sub.2 and H.sub.2 SO.sub.4, respectively, both at a concentration of 0.01 M. Thus, the total charge transported in the case of sulfuric acid was 2.75 times greater. The flow rates of the feed were similar. If the electrostatic mode alone were operative, the total charge transported would have to be similar.
With a solution of NaCl at a concentration of 0.001 M and at a low current of 17.5 mA, pulse length of up to 3.62 h was observed, which exceeds by far the time required to charge the electrodes capacitively to the maximum preset voltage.
(d) Evidence from Constant Levels of Deionization and Enhancement
The third piece of evidence can be seen again in FIG. 1, where nearly constant and similar levels of deionization and enhancement are maintained over the greater part of each pulse. This result is consistent with a constant, high `leakage current` arising from the sliding diffuse layers. Similarly, in the 0.001 M NaCl case above, constant levels of deionization and enhancement of about 50% and 150%, respectively, have been observed for over three hours in each pulse.
(e) Evidence from the First Successful Deionization Using the ELCOR.TM. Process Operating in the Electrostatic Mode
A complete discussion of this evidence is presented in the Section entitled "Example."
Process Parameters Affecting the Deionization Process
The following parameters have been identified as being likely to affect deionization in the ELDYN mode.
Centrifugal Force. This is the prime independent parameter expected to give rise to the ELDYN mode and to have profound, beneficial effects on the process current, rate of deionization, Faradaic and energy efficiency, and the ultimate level of water purity. The rate of sliding and removal of the densified, diffuse layer is expected to be directly proportional to the magnitude of the G force generated by the Coriolis force, which creates `outward` force on said layer, thereby setting it in sliding motion. The resulting continuous removal of the diffuse layer facilitates maintaining the state of charge or polarization of the electrodes at a low level and the voltage across the electrolyte at a high value. This condition, in turn, favors high current and faster ionic transport across the cell width.
Electric Field. The rate of ionic transport across the cell is directly proportional to the electric field, which is the second of two key parameters affecting the ELDYN mode. A maximum limiting voltage, just below the decomposition potential for the electrolyte (ca. 1.1 V), can be used for the process.
Another parameter for maximizing the electric field is the electrode spacing, as discussed further on.
Surface Area of the Electrodes. As in the case of the electrostatic mode, the HSA electrodes are a pre-requisite for maintaining high current density and, thereby, high rate of deionization. In combination with sufficient centrifugal force, HSA electrodes produce a condition of constant, high, dc current at a constant maximum voltage to facilitate a continuous operation without the need of changing polarity. Supercapacitor electrodes such as those described in the Hanak patent, employed in the electrostatic mode, can be used.
Flow Rate of the Feed Liquid. The flow rate of the feed liquid affects enhancement and depletion; the ratio of the two quantities is the separation ratio. To date the limit of this ratio for a single stage has not been established. Its magnitude is expected to determine the number of stages in a multi-stage device to achieve a desired degree of deionization and enhancement.
Ratio of the Effluent Flow Rates. In order to achieve a cost-effective disposal or recovery of the dissolved solute, it should be concentrated into a minimal practical volume. For this purpose, the ratio of the flow rate of the diluent and the concentrate (Rd/Rc) should be substantially greater than 1, perhaps as large as 10 or more. An additional benefit from the high ratio is a substantial increase in the volume of the purified diluent.
Concentration of the Solute. The concentration of the feed affects the efficiency of water-treatment processes. As reported in the Hanak patent, the range of concentration selected for the initial feed has been shown to span over a range of over three orders of magnitude, from 0.0001 to 0.3 M, corresponding to ca. 10 to 30,000 mg/L for selected solutes.
Temperature. Elevated temperature, by promoting molecular motion and lowering surface tension and intermolecular cohesion, may favor the ELDYN mode in a manner similar to electrolytic processes. Maintaining a constant temperature would minimize the effect of this variable.
Ionic Properties as Process Parameters
In addition to the preceding process parameters, the following materials' parameters are expected to affect deionization in this mode.
Ionic Mass. Ionic transport is inversely proportional to the ionic mass, slowing down the heavy ions. However, the rate of sliding and removal of the densified, diffuse ionic sheath should be also proportional to the ionic mass. In turn, it should help maintain a low state of polarization and high electric field, enhancing the transport of the heavy ions across the cell. Thus, high ionic mass should be an important factor in the deionization of heavy ions, such as those of the transuranic elements.
Ionic Radius. The transference number is inversely proportional to the ionic size, meaning slower transit between electrodes. This condition is in part compensated for by a lower state of electrode polarization resulting from a relatively lower population of the ions on the electrode surfaces because of their large size. Furthermore, large ionic size also results in diminished ionic charge density, which would promote sliding. Thus, on balance, large ionic size is expected to favor the ELDYN mode.
Ionic Valence. A major impact on ionic transport is that the transport current required is directly proportional to ionic valence. In addition, increased charge on multivalent ions should result in greater attraction to the electrode and possibly an increase in the `braking` action to sliding. On the other hand, greater charge on the cations leads to higher solvation, making the ion larger, with a resulting positive, offsetting effect discussed above.
Dependent Process Parameters
The dependent process parameters are the process current and the three conductivities for determining concentrations of the feed and the effluents. All are monitored in real time, along with the process voltage, an independent parameter. It should be noted that in the case of the electrolytic and electrostatic modes, the process current, set to a constant level, was an independent process parameter. In the ELDYN mode it is more advantageous for the current to be a parameter dependent on other variables such as centrifugal force and electric field.
TABLE 1 List of Computed Deionization Performance Parameters Acronym Parameter Units AVIP Average process current MA DSO Observed relative deionization % ONH Observed relative enhancement % DST Theoretical relative deionization % TNH Theoretical relative enhancement % FEFD Faradaic deionization efficiency % FEFC Faradaic enhancement efficiency % SEP Separation ratio (ONH/DSO) -- KGKWH rate of mass removal per unit of energy kg/kWh ENERW energy per unit volume of water desalinated kWh/m.sup.3 COSTW cost per unit volume of water desalinated $/m.sup.3 COSTCP cost per unit mass of chemical compound recovered $/kg COSTR cost per unit mass of metal ion or anion recovered $/kg
Computed Performance Parameters
Formulas and software have been developed for computing deionization performance parameters. The software provides for continuous monitoring of the independent and dependent parameters during the process. The performance data are based on the starting concentration of the feed, the observed concentrations of the effluents, and other dependent and independent process parameters. The concentrations can be determined conductometrically from the expression log C=a log K+b, where C is the concentration, K is the conductivity, and a and b are constants characteristic of each material. A temperature correction to a common temperature is accomplished automatically by the conductivity meter. A list of deionization parameters that are computed and tabulated automatically at the end of each waste-water-treatment run appears in Table 1. They serve as the data base for evaluating the technical and economic merits of the process.
Predicted High Efficiency of Deionization in the ELDYN Mode
When the ELCOR.TM. process can be made to operate predominantly in the ELDYN mode, by substantially increasing the centrifugal force, the energy efficiency, rate of deionization, and cost-effectiveness will rival those of any other process. Consequently, a system design and operational features anticipated for the ELDYN mode can be incorporated into the ELCOR.TM. process disclosed in the Hanak patent. The ultimate goal will be to develop the most efficient and cost-effective process for the remediation of water resources which have been adversely affected by environmental pollution--including toxic wastes and radionuclides. The process will be also equally suitable for the treatment of water containing high levels of naturally occurring dissolved solids such as deep-well or brackish water.
The basis for the predicted high efficiency of deionization is as follows. First of all, there is a set quantity of energy associated with the removal of a solute from the feed solution, which is equivalent for all demineralization processes. Hence, this energy will not be considered in the comparison of the ELCOR.TM. process with other processes. For the ELCOR.TM. process operating in the electrolytic and electrostatic modes, it has been demonstrated that the energy efficiency is equal to or exceeds those of reverse osmosis (RO) and of electrodialysis (ED), not taking into account the energy required to run the centrifuge. (For large systems, centrifugation is estimated to be a small fraction of the total energy.) The energy expended in the electrolytic mode is mainly the sum of the resistive loss in the electrolyte, I.sup.2 R, where I is the electric current and R is the electrical resistance of the process liquid, plus the energy consumed by the electroplating and stripping operations. In the electrostatic mode it is again the I.sup.2 R loss plus the energy consumed by the capacitive charging and discharging. The results also indicate that the electrostatic mode is more energy-efficient than the electrolytic mode. In the ELDYN mode at steady state, when additional charging is no longer occurring, the sole source of expended energy is the I.sup.2 R component (again ignoring centrifugation). The ions arriving at the electrodes are simply balanced by those leaving the electrodes by the sliding action due to the centrifugal force. Thus, in the absence of the electrochemical components of energy loss, the process is more efficient in the ELDYN mode and also more efficient than RO or ED.
Operating Procedure for Deionization in the ELDYN Mode
The existing operating procedure used in the electrostatic mode employs constant current, which is an independent parameter in the Hanak patent. In that method, switching of polarity occurs when the limiting voltage is reached. A new, improved procedure employs constant voltage as an independent parameter. The process current is now a parameter that is dependent directly on the electric field and indirectly on the centrifugal force. As stated above, it is anticipated that in the ELDYN mode the current will saturate at a constant level proportional to the electric field and the centrifugal force, in addition to the ionic concentration in the feed. The software for process control and for evaluation requires appropriate modifications to accommodate this change.
As detailed in the Section entitled "Example" the data in FIG. 6 indicate that in the first part of each pulse, at lower voltage, charging of the electrodes is predominantly taking place, meaning that the electrostatic mode prevails. In the second part of the pulse, at higher voltage, it is clear that the ELDYN mode predominates, judging from the emergence of extensive concentration. This sequence of occurrence of the electrostatic and the ELDYN modes suggests that the HSA electrodes need to be at least partially populated by ionic species in order for significant rate of sliding of the ionic sheath to take place. The logic of this conclusion follows from the fact that the initially thin ionic sheath is more strongly attracted to the oppositely charged electrode surface than the subsequent thicker sheath. In the latter, additional ionic species can slide with relative ease over ions of the same polarity which are attracted more strongly to the electrode surface. The partial population of the electrode surface occurs automatically upon the application of voltage to discharged electrodes; requiring no additional provision for the ELDYN mode to occur.
A need for periodic, infrequent change in polarity of the electrodes is anticipated in order to clean the electrodes possibly soiled by microscopic solid matter, attracted to the surface. The intervals between such polarity reversal might be hours, days or weeks, if at all, most likely dependent on the quality of the feed liquid.
It should be pointed out that operation at a constant voltage is used in the capacitive deionization taught by Joseph Farmer in prior art [Ref 3]. However, in that process the current is not constant; it rises to a high value upon initial application of the voltage, and decreases asymptotically to a very low value with time, whereupon the electrodes must be discharged and regenerated. With the decrease of current the rate of deionization also decreases. As already stated, in the ELCOR.TM. process using the ELDYN mode a high, constant level of current persists, with no need to discharge the electrodes or switch polarity except for optional, occasional cleaning. Thus, the performance characteristics of the instant invention are superior to those of the Farmer patent.